1. Field of the Invention
The present invention generally relates to an output buffer and a source driver using the same, and more particularly, to the output buffer that increases driving abilities of charging and discharging without additional power consumption.
2. Description of Related Art
A source driver is an important element in a liquid crystal display (LCD). The source driver mainly includes a shift register for controlling a data latch to receive a digital video signal from a data bus by timing control, a digital-to-analog converter for converting the digital video signal into an analog driving signal, an output buffer for enhancing a driving ability of the driving signal, and an output multiplexer for outputting the driving signal to pixels on a display panel for displaying images.
FIG. 1 is a circuit diagram of a conventional output buffer in the source driver. Referring to FIG. 1, the output buffer 100 includes an input stage 110, a charging output stage 120, and a discharging output stage 130. The input stage 110 controls the charging output stage 120 and the discharging output stage 130 according to signals at input nods Vi+ and Vi−, wherein the output buffer 100 is a unity gain buffer having the input terminal Vi− coupled to an output terminal Vout thereof. When the signal at the input terminal Vi+ is larger than the signal at the input terminal Vi−, an induced current I1 is decreased to conduct a transistor M6 and a transistor M8. The conducted transistor M6 forms a charging path to increase a voltage at the output terminal Vout, and the conducted transistor M8 increases a voltage at terminal N1 for making a transistor M10 not conduct. In addition, when the signal at the input terminal Vi+ is less than the signal at the input terminal Vi−, the induced current I1 is increased to make the transistors M6 and M8 not conduct. In the meanwhile, a transistor M9 conducted by a bias voltage Vb pulls low the voltage at terminal N1, and then conducts a transistor M10 to form a discharging path so as to decrease the voltage at the output terminal Vout.
With the increase of the operation frequency, the source driver may not have sufficient time to charge/discharge the output terminal Vout to a target voltage, and then deliver the target voltage to the pixels on display panel to orient liquid crystal corresponding to the pixels. Therefore, the output buffer 100 should increase driving abilities of charging and discharging to increase a slew rate of the driving signal. However, designers may increase the width-to-length ratios of transistors to increase a charging/discharging current of the output buffer 100, but more power consumption and layout area would be necessary. There should be a proper circuit design in the output buffer for increasing the driving abilities of charging and discharging.